import type { OperatorFunction } from '../types.js';
import { Observable, operate } from '@rxjs/observable';

/**
 * Emits a given value if the source Observable completes without emitting any
 * `next` value, otherwise mirrors the source Observable.
 *
 * <span class="informal">If the source Observable turns out to be empty, then
 * this operator will emit a default value.</span>
 *
 * ![](defaultIfEmpty.png)
 *
 * `defaultIfEmpty` emits the values emitted by the source Observable or a
 * specified default value if the source Observable is empty (completes without
 * having emitted any `next` value).
 *
 * ## Example
 *
 * If no clicks happen in 5 seconds, then emit 'no clicks'
 *
 * ```ts
 * import { fromEvent, takeUntil, interval, defaultIfEmpty } from 'rxjs';
 *
 * const clicks = fromEvent(document, 'click');
 * const clicksBeforeFive = clicks.pipe(takeUntil(interval(5000)));
 * const result = clicksBeforeFive.pipe(defaultIfEmpty('no clicks'));
 * result.subscribe(x => console.log(x));
 * ```
 *
 * @see {@link EMPTY}
 * @see {@link last}
 *
 * @param defaultValue The default value used if the source
 * Observable is empty.
 * @return A function that returns an Observable that emits either the
 * specified `defaultValue` if the source Observable emits no items, or the
 * values emitted by the source Observable.
 */
export function defaultIfEmpty<T, R>(defaultValue: R): OperatorFunction<T, T | R> {
  return (source) =>
    new Observable((destination) => {
      let hasValue = false;
      source.subscribe(
        operate({
          destination,
          next: (value) => {
            hasValue = true;
            destination.next(value);
          },
          complete: () => {
            if (!hasValue) {
              destination.next(defaultValue!);
            }
            destination.complete();
          },
        })
      );
    });
}
